1. Introduction

The global energy shortage has put great pressure on human being to look for the new energy resource. The solar energy is a kind of unique clean and sustainable energy resource as compared with other conventional ones such as the coal, oil, natural gas and so on [1]. Photon-electrical and photon-thermal conversions are two most feasible approaches in application of the solar energy [2]. By converting photons into electrons, the solar cell has the advantage of easier application in many fields, but has the limit due to its narrower absorption spectrum window with relatively high material and production cost. For the device using the photon-thermal conversion, the optimal structure can be designed to achieve both of high solar absorption α in the visible and near infrared region with respect to the solar radiance spectra and the low thermal emittance ε in the far infrared region. These are two main physical criteria used to characterize the high photo-thermal conversion efficiency η [3].

Different approaches have been tried to realize high and broadband solar absorption in the solar-thermal conversion by using the material with proper structures like cermet [4, 5] and black components [6, 7] realized all through graded-index film [8] and metal-based periodic microstructures [9,10], and so on. Some of the structures show low thermal emittance to be practically put into application, but encounter difficulties to precisely and consistently control the composition of the material in the gas flow and chemical reaction process to produce the high quality graded-index structures.

An interference coating [2] had been cited with the structure of dielectric/metal stacks to achieve a spectral reflectance through chemical vapor deposition, but without more detailed information given in previous works. In this work, we are going to carry out such few layers of dielectric/metal thin films to realize high photo-thermal conversion efficiency with more feasible production procedure. By best matching of the optical constant and layer thickness of the pure dielectric and metal films, we have designed and realized the structure to achieve high optical absorptance (A) with both low reflectance (R) and transmittance (T) in the broad solar spectral region to satisfy the energy conversation condition of that (R+T) +A=1, while keeping the low thermal emittance in the infrared region.

2. Structure design

By properly choosing the high-optically absorbing material like the metal for the structure containing the dielectric layers to match the phase and amplitude of the light wave propagated in the material, the solar-thermal device can be optimally designed to achieve maximum absorption of the solar energy in the main solar radiance spectral region. We designed a 4-layer (dielectric/metal/dielectric/metal) structure on a Si substrate as shown in Fig. 1. From the bottom layer up to the top, the thick metal layer (Al or Ag, etc) has the high optical reflection property in the entire visible region. The next dielectric layer (SiO₂) will have the proper optical constant and thickness to match the phase and amplitude of solar wave propagated in the film. The solar energy layer (Ti, W, Cr, or other metals and alloys with the optical constant to be tunable for the optical properties of the device) is the key thin metal layer with the complex refractive index and thickness to realize the maximum energy absorption matching to the solar radiance spectrum. The top dielectric layer (SiO₂) is used to protect the device in variety of environment conditions.

 

Fig. 1. Sketch of the 4-layer (dielectric/metal/dielectric/metal) structure of solar-thermal conversion structure on the Si substrate.

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In terms of the structure and optical constants of the materials [11], the optical transfer-matrix method was used to simulate the optical properties of the device by calculating the reflectance, transmittance and absorptance of the structure in advance. The detailed parameters used in the simulation are as follows: the top layer SiO₂ (90∼110nm) is to have the function of protection and antireflection; the second Ti metal layer (10∼30nm) is to play the key role of high solar energy absorption; the third one is SiO₂ layer (80∼120nm), which is to perform the phase and amplitude matching function; the bottom Al (or Ag) layer (≥100nm) is to provide high optical reflection and almost zero transmission in the 400–1000nm wavelength region. The simulated results at normal incidence are shown in Fig. 2. It can be seen that over 95% of optical absorption can be reached in the 450-1000nm region ( > 90% in 420–1200nm region) as match to the solar radiance spectrum. The steep decrease of the absorption initiated at the wavelength λ ≈350nm (about 3.5eV) is due to the mismatching of the optical constants of layered materials in the region.

 

Fig. 2. Simulated data with respect to transmittance, reflectance and absorptance of the 4-layer structure [SiO₂(105nm)/Ti(15nm)/SiO₂(95nm)/Al(100nm)] on the Si substrate at normal incidence as compared with the solar irradiance spectrum [12].

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3. Experiment

The multilayered thin film sample was prepared by sputtering in the Leybold LAB600SP chamber in room temperature. The metal and dielectric films with the target purity of 99.99% were deposited by DC and RF magnetron on the Si substrate, respectively with a background pressure of 6.0×10^-6 mbar. The thickness of the film is controlled by deposition time according to the sputtering rate which was determined by the power and flux of Argon gas flow rate as calibrated by the ellipsometer and Kosaka Surfcorder ET300 in advance.

Although Cr, W and other metal materials can be chosen as the energy absorption layer, we find that it will be more efficient for Ti to perform the high and uniform solar energy absorption as match to the solar radiance spectrum. Although either Al or Ag can be the candidate of the optical refraction layer, Al is superior with higher reflection extending to the ultraviolet region and costs less in production. The pure dielectric SiO₂ layer is transparent and stable in the environment with the optical refractive index less variable in the 200–1000nm wavelength region, and then it is suitable to use as the protection and interference layer for the device.

The spectra of the reflectance R [R=(R_s+R_p)/2, where R_s and R_p is the polarized reflectance normal and parallel to the incidence plane, respectively] and transmittance T (T ≈ 0) were measured by a variable- incidence-angle spectrometer in the 300–800nm wavelength region with the incidence angle changing from 35° to 80°. In terms of the spectral data of T and R, the absorptance A was reduced according to the conversation of energy A =1-R-T in the data analysis afterwards.

4. Results and discussions

We have measured the dependence of incident angle from 35° to 80° with a step of 5° for a typical layered structure [SiO₂(105nm)/Ti(15nm)/SiO₂(95nm)/Al(≥100nm)]. The measured and simulated spectra of absorption are shown in Figs. 3 and 4, respectively. In general the higher absorptance can be achieved at the small incidence angle condition; it decreases with the increasing of the incident angle. Taking the absorption at 35° as an example, the measured spectrum is in close agreement with the simulated one to show high absorptance over 90% in the 350–800nm region. Especially, the highest absorption efficiency of more than 99% has been reached in the deep blue region. Although the absorptance decreases with the incidence angle, experimental data show that it still has the value higher than 50% almost in the entire spectral region even at higher incidence angle of 80°.

 

Fig. 3. The measured spectra of absorptance changing with the incident angle for the layered structure of SiO₂(105nm)/Ti(15nm)/SiO₂(95nm)/Al( > 100nm).

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Fig. 4. The simulated spectra of absorptance changing with the incident angle based on the layered structure given in Fig. 3.

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In the study of the Al-based multilayer structure, we also tried to change the thickness of the Ti layer and found that the absorptance is not particularly sensitive to the slight variation of the Ti layer thickness in the spectral region. The results are shown in Fig. 5.

 

Fig. 5. The measured spectra of absorptance changing with the thickness of the Ti layer under the condition in which the thicknesses of the SiO₂ and Al layers are fixed and other parameters are the same as that given in Fig. 3.

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According to Planck’s black-body radiation E(T, λ), the thermal emittance ε can be calculated based on the spectra of reflectance R(θ, λ), especially in the long wavelength region by using the equation [3]:

We calculated the reflectance of the structure in the infrared region (1-32μm) by using the transfer-matrix method at normal incidence condition to obtain the thermal emittance which is 0.063 at 600K with a comparison to that at different incident angles as shown in the inset of Fig. 6. We also measured the reflectance R in the 1-5 μm infrared region at the incidence angle of 35° with the result given in Fig. 6 to show a good agreement with the simulated one which indicates a very low thermal emittance for the solar-to-thermal convert structure of the device.

 

Fig. 6. The comparison of reflectance spectra between measured and simulated one in the 1-5 μm infrared region for the structure given in Fig. 3. The inset shows the calculated emittance with respect to different incident angles.

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5. Conclusion

We have designed and achieved a multilayer structure which has high solar absorption efficiency (>95%) in the 400–1000nm wavelength region and low thermal emittance (ε=0.063 at 600K and normal incidence condition). The materials used in application are cost-effective with relatively simple procedure for production and quality control to show the potential application of the structure in the solar energy field.